I've migrated a number of large accounts through SurgeMail's built-in migration facility with good results overall.

Unfortunately, many of the users on my legacy system have their email clients set to use CRAM-MD5 authentication, which means SurgeMail doesn't receive the actual account passwords, ao it can't migrate the accounts automatically on demand.

It would be nice if we could provide SurgeMail with a list of passwords ahead of time to enable migration for CRAM-MD5 users.

Anyway, something to bear in mind.